Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters (e.g. diameters may range from 0.05 m up to 0.6 m). Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including polymer, and/or metallic, and/or composite layers. For example, a pipe body may include polymer and metal layers, or polymer and composite layers, or polymer, metal and composite layers.
In many known flexible pipe designs the pipe body includes one or more tensile armour layers. The primary loading on such a layer is tension. In high pressure applications, such as in deep and ultra deep water environments, the tensile armour layer experiences high tension loads from a combination of the internal pressure end cap load and the self-supported weight of the flexible pipe. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time. That is, in deep and ultra-deep water environments, the weight of the pipe itself causes a high tension loading on the pipe, which will be greatest at the hang off region (where the pipe attaches to a vessel or floating facility).
FIG. 4 illustrates a riser assembly 400 suitable for transporting production fluid such as oil and/or gas and/or water from a sub-sea location 401 to a floating facility. For example, in FIG. 4 the sub-sea location 401 includes a sub-sea flow line. The flexible flow line 405 comprises a flexible pipe, wholly or in part, resting on the sea floor 404 or buried below the sea floor and used in a static application. The floating facility may be provided by a platform and/or buoy or, as illustrated in FIG. 4, a ship 402. The riser assembly 400 is provided as a flexible riser, that is to say a flexible pipe 403 connecting the ship to the sea floor installation. The flexible pipe may be in segments of flexible pipe body with connecting end fittings.
It will be appreciated that there are different types of riser, as is well-known by those skilled in the art. Embodiments of the present invention may be used with any type of riser, such as a freely suspended (free, catenary riser), a riser restrained to some extent (buoys, chains), totally restrained riser or enclosed in a tube (I or J tubes).
FIG. 4 also illustrates how portions of flexible pipe can be utilised as a flow line 405 or jumper 406.
One technique which has been attempted in the past to in some way alleviate the above-mentioned problem is the addition of buoyancy aids at predetermined locations along the length of a riser. The buoyancy aids provide an upwards lift to counteract the weight of the riser, effectively taking a portion of the weight of the riser, at various points along its length. Employment of buoyancy aids involves a relatively lower installation cost compared to some other configurations, such as a mid-water arch structure, and also allows a relatively faster installation time.
An example of a known riser configuration using buoyancy aids to support the riser is a stepped riser configuration 100, such as disclosed in WO2007/125276 and shown in FIG. 1, in which buoyancy aids 101 are provided at discrete locations along a flexible pipe 103. The riser is suitable for transporting production fluid such as oil and/or gas and/or water from a subsea location to a floating facility 105 such as a platform or buoy or ship. In some cases this assembly may restrict the amount of vessel excursion permitted.
Other riser configurations may require the addition of ballast weight to a flexible pipe to decrease the buoyancy of the pipe at one or more positions to suit a particular marine environment or production fluid extraction set up.
As used herein, the term “buoyancy compensating element” is used to encompass both buoyancy aids for increasing buoyancy and ballast weights for decreasing buoyancy. The term “buoyancy aid” is used to encompass elements for increasing buoyancy and the term “ballast weight” is used to encompass elements for decreasing buoyancy.
As used herein, when discussing a portion of riser that is “below” or “lower than” another portion, the portion “below” or “lower” is further along the riser in the direction of the seabed. Similarly, when discussing a portion of the riser that is “above” or “higher than” another portion, the portion “above” or “higher” is further along the riser in the direction of the sea surface.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 meters)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. For example, a flexible pipe may be required to operate with external pressures ranging from 0.1 MPa to 30 MPa acting on the pipe. Equally, transporting oil, gas or water may well give rise to high pressures acting on the flexible pipe from within, for example with internal pressures ranging from zero to 140 MPa from bore fluid acting on the pipe. As a result the need for high levels of performance from the layers of the flexible pipe body is increased.
The end fittings of a flexible pipe may be used for connecting segments of flexible pipe body together or for connecting them to terminal equipment such as a rigid sub-sea structures or floating facilities. As such, amongst other varied uses, flexible pipe can be used to provide a riser assembly for transporting fluids from a sub-sea flow line to a floating structure. In such a riser assembly a first segment of flexible pipe may be connected to one or more further segments of flexible pipe. Each segment of flexible pipe includes at least one end fitting.
FIGS. 2a and 2b illustrate a portion of a known riser configuration suitable for deep and ultra-deep water. As shown in FIG. 2a, a plurality of buoyancy modules 201 are connected to the riser 200 in an in-line configuration. This is known in the art as a mid-line buoyancy system. The buoyancy modules 201 help to control hang off tension level in the riser. However, in use the riser is subjected to dynamic loading due to vessel motion or tidal effects, for example, which can cause curvature changes in the riser configuration. Overbending can also occur when the flexible pipe is installed. It is generally advantageous to prevent overbending and control such changes within predetermined limits. In this example, the riser between the mid-line buoyancy system and the vessel hang off may be subject to slack and subsequent compression and bending at the top of the mid-line buoyancy system as shown in circle A.
For deep water, the vessel offset (the degree of vertical movement of the vessel) may be as large as 10-12% of the water depth. As such, for ultra-deep water (for example 2200 m), the vessel offset may be up to 264 m. This degree of movement can cause significant bending and compression of the riser, which can lead to riser fatigue.
Previously, when providing be compensating elements in an in-line configuration, as disclosed in WO2013/079915, the top first buoyancy module 202 has been designed with a bellmouth profile 204 (as shown in FIG. 2b), to help control the bending of the pipe as it exits the mid-line buoyancy system, to prevent the riser from compression and overbending. However, the bellmouth profile 204 may not be suitable for all applications. Furthermore, the profiled buoyancy module 202 requires additional design and manufacture effort.
It would be useful to provide a riser assembly that is suitable for deep and ultra-deep water environments that overcomes or ameliorates the various issues mentioned above.